Merge pull request #30168 from sshivensolanki:fix-trackervit-scalefactor

video: fix TrackerVit input scalefactor computed as Scalar - #30168

Fixes #30164

### Problem

`TrackerVitImpl` computes the input normalisation as

    i2bp.scalefactor = (1.0 / parameters.stdvalue) * (1 / 255.0);

`operator/(double, Scalar)` is the quaternion inverse (`conj(b) * a / |b|^2`), not an elementwise reciprocal. With the default `stdvalue = (0.229, 0.224, 0.225)` the applied scalefactor is `(0.00586, -0.00573, -0.00576)` instead of `(0.01712, 0.01751, 0.01743)`: channels 1 and 2 are sign-flipped and all three are about 1/3 of the intended magnitude. This was introduced in 4.11 when the elementwise `(dst - mean) / std` was replaced with `Image2BlobParams`.

### Fix

Compute `1 / (255 * std)` per channel in a small helper used by both constructors. A zero std entry (the unused 4th channel by default) maps to 0.

### Tests

`vittrack.accuracy_vittrack` on the `tracking/bag` sequence, measured locally on macOS ARM64 (4.x and 5.x):

|  | min IoU | mean IoU |
|---|---|---|
| before | 0.642 | 0.757 |
| after | 0.682 | 0.776 |

On other CI platforms (x64, Windows, no-HAL) the minimum IoU with the fix is 0.658, so the existing 0.64 threshold is kept.

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/video/src/tracking/tracker_vit.cpp

@@ -38,6 +38,15 @@ TrackerVit::Params::Params()
 
 #ifdef HAVE_OPENCV_DNN
 
+// Per-channel 1/(255*std). Note: (1.0 / Scalar) is the quaternion inverse, not an elementwise reciprocal
+static Scalar getScaleFactor(const Scalar& stdvalue)
+{
+    Scalar scalefactor;
+    for (int i = 0; i < 4; i++)
+        scalefactor[i] = stdvalue[i] != 0 ? 1.0 / (stdvalue[i] * 255.0) : 0.0;
+    return scalefactor;
+}
+
 class TrackerVitImpl : public TrackerVit
 {
 public:
@@ -50,7 +59,7 @@ public:
         net.setPreferableTarget(parameters.target);
 
         i2bp.mean = parameters.meanvalue * 255.0;
-        i2bp.scalefactor = (1.0 / parameters.stdvalue) * (1 / 255.0);
+        i2bp.scalefactor = getScaleFactor(parameters.stdvalue);
         tracking_score_threshold = parameters.tracking_score_threshold;
     }
 
@@ -60,7 +69,7 @@ public:
 
         net = model;
         i2bp.mean = meanvalue * 255.0;
-        i2bp.scalefactor = (1.0 / stdvalue) * (1 / 255.0);
+        i2bp.scalefactor = getScaleFactor(stdvalue);
         tracking_score_threshold = _tracking_score_threshold;
     }